MDEV-28534: clang-12 compile warnings

Errors where:

/buildbot/amd64-ubuntu-2004-msan/build/sql/item.h:6478:12: error: 'val_datetime_packed' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  longlong val_datetime_packed(THD *thd)
           ^
/buildbot/amd64-ubuntu-2004-msan/build/sql/item.h:3501:12: note: overridden virtual function is here
  longlong val_datetime_packed(THD *thd) override;
           ^
/buildbot/amd64-ubuntu-2004-msan/build/sql/item.h:6480:12: error: 'val_time_packed' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  longlong val_time_packed(THD *thd)
           ^
/buildbot/amd64-ubuntu-2004-msan/build/sql/item.h:3502:12: note: overridden virtual function is here
  longlong val_time_packed(THD *thd) override;
           ^

sql/item.h

@@ -6475,9 +6475,9 @@ class Item_default_value : public Item_field
   bool get_date(THD *thd, MYSQL_TIME *ltime,date_mode_t fuzzydate) override;
   bool val_native(THD *thd, Native *to) override;
   bool val_native_result(THD *thd, Native *to) override;
-  longlong val_datetime_packed(THD *thd)
+  longlong val_datetime_packed(THD *thd) override
   { return Item::val_datetime_packed(thd); }
-  longlong val_time_packed(THD *thd)
+  longlong val_time_packed(THD *thd) override
   { return Item::val_time_packed(thd); }
 
   /* Result variants */